The mass accretion rate required to power the disc emission is
comparable with that required to power the coronal hard X-ray emission
assuming that the corona extends down to a radius which is \(\sim 5\) 
times smaller than that of the disc i.e. \(3.5 R_g\)  for the parameters
assumed here, i.e. \(\sim 9~R_g\)  with a colour temperature correction.
We stress again that these system parameters are poorly known, thought
this appears quite reasonable. Much smaller radii could be potentially
feasible for a hot inner flow, irrespective of the spin of the black
hole, as numerical simulations of the MRI turbulence show that the
large scale height magnetic fields can extract energy from the
infalling material beyond the last stable orbit
(e.g. [1]}).
